Mullite-containing carrier for ethylene oxide catalysts

ABSTRACT

The present invention relates to an improved carrier for an ethylene epoxidation catalyst, the carrier comprising alumina in combination with a stability-enhancing amount of mullite. The invention is also directed to an improved catalyst containing the improved carrier, as well as an improved process for the epoxidation of ethylene using the catalyst of the invention.

RELATED APPLICATIONS

The present application is a divisional of co-pending application havingU.S. Ser. No. 12/360,457, filed on Jan. 27, 2009, which claims benefitof U.S. Provisional Application Ser. No. 61/082,016, filed Jul. 18,2008, the entire contents of which are incorporated herein by reference.

FIELD OF THE INVENTION

The present invention relates to silver-based ethylene oxide catalysts,and more particularly, to carriers for such catalysts.

BACKGROUND OF THE INVENTION

As known in the art, high selectivity catalysts (HSCs) for theepoxidation of ethylene refer to those catalysts that possessselectivity values higher than high activity catalysts (HACs) used forthe same purpose. Both types of catalysts include silver as the activecatalytic component on a refractory support (i.e., carrier). Typically,one or more promoters are included in the catalyst to improve or adjustproperties of the catalyst, such as selectivity.

Generally, HSCs achieve the higher selectivity (typically, in excess of87 mole % or above) by incorporation of rhenium as a promoter.Typically, one or more additional promoters selected from alkali metals(e.g., cesium), alkaline earth metals, transition metals (e.g., tungstencompounds), and main group metals (e.g., sulfur and/or halide compounds)are also included.

There are also ethylene epoxidation catalysts that may not possess theselectivity values typically associated with HSCs, though theselectivity values are improved over HACs. These types of catalysts canalso be considered within the class of HSCs, or alternatively, they canbe considered to belong to a separate class, e.g., “medium selectivitycatalysts” or “MSCs.” These types of catalysts typically exhibitselectivities of at least 83 mole % and up to 87 mole %.

In contrast to HSCs and MSCs, the HACs are ethylene epoxidationcatalysts that generally do not include rhenium, and for this reason, donot provide the selectivity values of HSCs or MSCs. Typically, HACsinclude cesium (Cs) as the only promoter.

It is well known that with use of a catalyst, the catalyst will age(i.e., degrade) to a point until use of the catalyst is no longerpractical. For obvious reasons, there is a continuous effort to extendthe useful lifetime (i.e., “longevity” or “usable life”) of catalysts.The useful lifetime of the catalyst is directly dependent on thestability of the catalyst. As used herein, the “useful lifetime” is thetime period for which a catalyst can be used until one or morefunctional parameters, such as selectivity or activity, degrade to sucha level that use of the catalyst becomes impractical.

It is known in the art that, while the selectivity of HSCs is generallyacceptable to the industry, their useful lifetime can use improvement.For example, while HACs typically last between 24 and 36 months, HSCstend to be operated for less than 24 months, often less than 12 months,typically due to an unacceptable loss of selectivity.

Stability of the catalyst has largely been attributed to variouscharacteristics of the carrier. Some characteristics of the carrier thathave undergone much research include surface area, porosity, and porevolume distribution, among others.

The most widely used formulation for the carriers of ethyleneepoxidation catalysts are those based on alumina, typically a-aluminaMuch research has been directed to investigating the effect of thealumina composition for improving stability and other properties of thecatalyst. The preparation and modification of alumina carriers forenhancing ethylene epoxidation catalyst performance are described, forexample, in U.S. Pat. Nos. 4,226,782, 4,242,235, 5,266,548, 5,380,697,5,597,773, 5,831,037 and 6,831,037 as well as in U.S. Patent ApplicationPublication Nos. 2004/0110973 A1 and 2005/0096219 A1. In particular,U.S. Pat. No. 5,395,812 discloses coating the outer surface and surfaceof pores therein with an amorphous silica-alumina mixture in order toimprove, inter alia, the lifetime of a silver-based ethylene epoxidationcatalyst.

However, there remains a need in the art for further improvements in thestability of ethylene epoxidation catalysts. There is a particular needfor improving the stability of such catalysts by modifying the carrierby means that are facile and financially feasible.

SUMMARY OF THE INVENTION

The present invention provides alumina carriers useful for preparingHSCs having an increased stability by incorporation therein of astability-enhancing amount of mullite.

The invention is also directed to a stability-enhanced ethyleneepoxidation catalyst comprising the stability-enhancing carrierdescribed above, along with a catalytic amount of silver and a promotingamount of rhenium deposited on and/or in the carrier. The increasedstability results in HSC (and MSC) catalysts with longer usablelifetimes, and particularly, catalysts that exhibit a significantlyreduced degradation in selectivity as compared to such catalysts withoutthe incorporation of mullite, over equivalent time periods of usage.

The invention is also directed to a method for the vapor phaseconversion of ethylene to ethylene oxide (EO) in the presence of oxygen,the method comprising reacting a reaction mixture comprising ethyleneand oxygen in the presence of the stability-enhanced ethyleneepoxidation catalyst described above.

The invention provides a stability-enhanced ethylene epoxidationcatalyst which is advantageously more resistant to degradation andretains a higher level of selectivity over time than similar catalyststhat have not been stability-enhanced in accordance with the inventiondescribed herein. The invention is thus highly beneficial in that thelonger catalyst life amounts to significant financial savings, greaterefficiency of the process, and less process and catalyst waste.

DETAILED DESCRIPTION OF THE INVENTION

In one aspect, the invention is directed to an improved alumina carrierfor an ethylene epoxidation catalyst. The carrier is improved in that itimparts an enhanced stability to a silver-based catalyst derivedtherefrom.

The carrier (i.e., support) provides this enhanced stability by havingincorporated therein a stability-enhancing amount of mullite within thealumina. As used herein, “mullite” (also known as “porcelainite”) refersto an aluminum silicate mineral having an Al₂O₃ component combined as asolid solution with a SiO₂ phase, wherein the Al₂O₃ component is presentin a concentration of at least about 40 mole percent and typically up toabout 80 mole percent. More typically, mullite contains the Al₂O₃component in a concentration of 60±5 mole percent, which can thus beapproximately represented by the formula 3Al₂O₃.2SiO₂ (i.e., Al₆Si₂O₁₃).

Since natural sources of mullite are scarce, most commercial sources ofmullite are synthetic. A variety of synthetic methods are known in theart. In one embodiment, the mullite used contains no other componentother than the alumina and silica components described above, except forone or more components that may be present in trace amounts (e.g., lessthan 0.1 mole or weight percent). In another embodiment, the mulliteused can include one or more additional components. For example, sodiumoxide (Na₂O) can be included in a minor amount (typically no more thanabout 1.0 mole or weight percent). Other components, such as zirconia(Zr₂O) or silicon carbide (SiC) can be included to, for example,increase fracture toughness. Numerous other metal oxides can also beincorporated to alter the properties of the mullite.

A stability-enhancing amount of mullite is typically at least about 0.5%and up to about 20% of mullite by weight of the carrier. In oneembodiment, the mullite is present in the carrier in a concentration ofat least about 1 wt % and up to about 20 wt %, 15 wt %, 12 wt %, 10 wt%, 8 wt %, 6 wt %, 5 wt %, 4 wt %, 3 wt %, or 2 wt % of the carrier. Inanother embodiment, the mullite is present in the carrier in aconcentration of at least about 3 wt % and up to about 20 wt %, 15 wt %,12 wt %, 10 wt %, 8 wt %, 6 wt %, 5 wt %, or 4 wt % of the carrier. Inyet another embodiment, the mullite is present in the carrier in aconcentration of at least about 5 wt % and up to about 20 wt %, 15 wt %,12 wt %, 10 wt %, 8 wt %, 7 wt %, or 6 wt % of the carrier. In stillanother embodiment, the mullite is present in the carrier in aconcentration of at least about 7 wt % and up to about 20 wt %, 15 wt %,12 wt %, 10 wt %, 9 wt %, or 8 wt %. In still other embodiments, themullite can be present in the carrier within a concentration range ofabout 0.5-15 wt %, 0.5-12 wt %, 0.5-10 wt %, 0.5-8 wt %, 0.5-6 wt %,0.5-5 wt %, 0.5-3 wt %, 0.5-2 wt %, 10-20 wt %, or 10-15 wt %.

In one embodiment, the outer surface of the alumina carrier is coatedwith mullite. The outer surface may be coated in conjunction withsubsurface or interior portions of the carrier also containing mullite,or alternatively, in the absence of either subsurface or interiorportions containing mullite.

In another embodiment, the outer surface of the alumina carrier is notcoated with mullite while either a subsurface or interior region of thecarrier contains mullite.

The carriers of the invention are composed of mullite and any of therefractory alumina compositions known in the art for use in ethyleneoxidation catalysts. However, the preferred carriers are based onalpha-alumina and mullite. Typically, the catalyst is composed ofalpha-alumina and mullite particles that are bonded together by abonding agent. The alpha-alumina used in the inventive carrierpreferably has a very high purity, i.e., about 95% or more, and morepreferably, 98 wt. % or more alpha-alumina Remaining components may beother phases of alumina, silica, alkali metal oxides (e.g., sodiumoxide) and trace amounts of other metal-containing and/ornon-metal-containing additives or impurities. Suitable aluminacompositions are manufactured and/or commercially available from, forexample, Noritake of Nagoya, Japan, and the NorPro Company of Akron,Ohio.

In general, a suitable catalyst carrier of the present invention can beprepared by combining the alumina, mullite, a solvent such as water, atemporary binder or burnout material, a permanent binder, and/or aporosity controlling agent, and then firing (i.e., calcining) themixture by methods well known in the art.

Temporary binders, or burnout materials, include cellulose, substitutedcelluloses, e.g., methylcellulose, ethylcellulose, andcarboxyethylcellulose, stearates (such as organic stearate esters, e.g.,methyl or ethyl stearate), waxes, granulated polyolefins (e.g.,polyethylene and polypropylene), walnut shell flour, and the like, whichare decomposable at the temperatures employed. The binders areresponsible for imparting porosity to the carrier material. Burnoutmaterial is used primarily to ensure the preservation of a porousstructure during the green (i.e., unfired phase) in which the mixturemay be shaped into particles by molding or extrusion processes. Burnoutmaterials are essentially completely removed during the firing toproduce the finished carrier.

The carriers of the invention are preferably prepared with the inclusionof a binder material in sufficient amount to substantially prevent theformation of crystalline silica compounds. Permanent binders include,for example, inorganic clay-type materials, such as silica and an alkalimetal compound. A convenient binder material which may be incorporatedwith the alumina particles is a mixture of boehmite, anammonia-stabilized silica sol, and a soluble sodium salt.

The formed paste is extruded or molded into the desired shape and firedat a temperature typically from about 1200° C. to about 1600° C. to formthe carrier. Where the particles are formed by extrusion, it may bedesirable to include conventional extrusion aids. Generally, theperformance of the carrier is enhanced if it is treated by soaking thecarrier in a solution of an alkali hydroxide such as sodium hydroxide,potassium hydroxide, or an acid such as HNO₃ as described in U.S. PatentApplication Publication No. 2006/0252643 A1. After treatment, thecarrier is preferably washed, such as with water, to remove unreacteddissolved material and treating solution, and then optionally dried.

The carrier of the invention is preferably porous and typically has aB.E.T. surface area of at most 20 m²/g. The B.E.T. surface area is moretypically in the range of about 0.1 to 10 m²/g, and more typically from1 to 5 m²/g. In other embodiments, the carriers of the invention arecharacterized by having a B.E.T. surface area from about 0.3 m²/g toabout 3 m²/g, preferably from about 0.6 m²/g to about 2.5 m²/g, and morepreferably from about 0.7 m²/g to about 2.0 m²/g. The B.E.T. surfacearea described herein can be measured by any suitable method, but ismore preferably obtained by the method described in Brunauer, S., etal., J. Am. Chem. Soc., 60, 309-16 (1938). The final support typicallypossesses a water absorption value ranging from about 0.2 cc/g to about0.8 cc/g, and more typically from about 0.25 cc/g to about 0.6 cc/g.

The carrier can have any suitable distribution of pore diameters. Asused herein, the “pore diameter” is used interchangeably with “poresize”. Typically, the pore diameters are at least about 0.01 microns(0.01 μm), and more typically, at least about 0.1 μm. In differentembodiments, the pore diameters can be at least about 0.2 μm, or 0.3 μm,or 0.4 μm, or 0.5 μm, or 0.6 μm, or 0.7 μm, or 0.8 μm, or 0.9 μm, or 1.0μm, or 1.5 μm, or 2.0 μm. Typically, the pore diameters are no more thanabout 50 μm, 40 μm, 30 μm, 20 μm, or 10 μm. In particular embodiments,the pore diameters are no more than about 9 μm, or 8 μm, or 7 μm, or 6μm, or 5 μm, or 4 μm, or 3 μm, or 2.5 μm. Any range derived from theforegoing minimum and maximum exemplary values is also suitable herein.In different embodiments, the suitable pore diameter range can be, forexample, any one of 0.01-50 μm, 1-50 μm, 2-50 μm, 5-50 μm, 10-50 μm,20-50 μm, 30-50 μm, 0.01-40 μm, 1-40 μm, 2-40 μm, 5-40 μm, 10-40 μm,20-40 μm, 30-40 μm, 0.01-30 μm, 0.05-30 μm, 0.1-30 μm, 0.5-30 μm, 1-30μm, 2-30 μm, 3-30 μm, 4-30 μm, 5-30 μm, 10-30 μm, 15-30 μm, 20-30 μm,0.01-10 μm, 0.05-10 μm, 0.1-10 μm, 0.5-10 μm, 1-10 μm, 2-10 μm, 3-10 μm,4-10 μm, 5-10 μm, 6-10 μm, 7-10 μm, 8-10 μm, 9-10 μm, 0.01-8 μm, 0.05-8μm, 0.1-8 μm, 0.5-8 μm, 1-8 μm, 1.5-8 μm, 2-8 μm, 2.5-8 μm, 3-8 μm, 4-8μm, 5-8 μm, 6-8 μm, 7-8 μm, 0.01-6 μm, 0.05-6 μm, 0.1-6 tun, 0.5-6 μm,1-6 μm, 1.5-6 μm, 2-6 μm, 2.5-6 μm, 3-6 μm, 4-6 μm, 5-6 μm, 0.01-5 μm,0.05-5 μm, 0.1-5 μm, 0.5-5 μm, 1-5 μm, 1.5-5 μm, 2-5 μm, 2.5-5 μm, 3-5μm, 3.5-5 μm, 4-5 μm, 0.01-4 μm, 0.05-4 μm, 0.1-4 μm, 0.5-4 μm, 1-4 μm,1.5-4 μm, 2-4 μm, 2.5-4 μm, 3-4 μm, 3.5-4 0.01-3 μm, 0.05-3 μm, 0.1-3μm, 0.5-3 μm, 1-3 μm, 1.5-3 2-3 μm, 2.5-3 μm, 0.01-2 μm, 0.05-2 μm,0.1-2 82 m, 0.5-2 μm, 1-2 μm, and 1.5-2 μm, as long as the range of eachmode of pores is different and each range possesses a different poresize of maximum concentration.

In a particular embodiment, the carrier possesses a multimodal pore sizedistribution (i.e., different pore size ranges, each range possessing adifferent pore size of maximum concentration). The multimodal pore sizedistribution is at least bimodal, and can thus be trimodal, tetramodal,or of a higher modality. The multimodal pore size distribution ischaracterized by the presence of at least two distributions (modes) ofpore sizes, each pore size distribution being either overlapping ornon-overlapping with another pore size distribution, and each pore sizedistribution having its own range of pore sizes (pore diameters) andpeak concentration (typically expressed as peak pore volume). Each poresize distribution can be characterized by a single mean pore size (meanpore diameter) value. Accordingly, a mean pore size value given for apore size distribution necessarily corresponds to a range of pore sizesthat result in the indicated mean pore size value.

The first mode and second mode of pores possess different mean poresizes (i.e., different mean pore diameters). Preferably, at least one ofthe modes of pores has a mean pore diameter within the range of about0.01 μm to about 5 μm. More preferably, both a first and second mode ofpores have a mean pore diameter within the range of about 0.01 μm toabout 5 μm as long as the mean pore diameters are different. Forexample, at least one of the first and second mode of pores can have amean pore size of about 0.01 μm, 0.02 μm, 0.03 μm, 0.04 μm, 0.05 μm,0.06 μm, 0.07 μm, 0.08 μm, 0.09 μm, 0.1 μm, 0.2 μm, 0.3 μm, 0.4 μm, 0.5μm, 0.6 0.7 μm, 0.8 μm, 0.9 μm, 1.0 μm, μm, 1.2 μm, 1.3 μm, 1.4 μm, 1.5μm, 1.6 μm, 1.7 μm, 1.8 μm, 1.9 μm, 2.0 μm, 2.1 μm, 2.2 μm, 2.3 μm, 2.4μm, 2.5 μm, 2.6 μm, 2.7 μm, 2.8 μm, 2.9 μm, 3.0 μm, 3.1 μm, 3.2 μm, 3.3μm, 3.4 μm, 3.5 μm, 3.6 μm, 3.7 μm, 3.8 μm, 3.9 μm, 4.0 μm, 4.1 μm, 4.2μm, 4.3 μm, 4.4 μm, 4.5 μm, 4.6 μm, 4.7 μm, 4.8 μm, 4.9 μm, or 5.0 μm.Two or more modes of pores can also be independently selected from anyof the above mean pore sizes as long as the mean pore sizes for eachmode of pores are different. Any range derived from any two valuesrecited above are also contemplated herein.

In another embodiment, at least one mode of pores is characterized byhaving a mean pore diameter above 5 μm up to about 30 μm. For example,in different embodiments, at least one mode of pores can have a meanpore diameter above 5 μm to about 25 μm, or above 5 μm to about 20 μm,or above 5 μm to about 15 μm, or above 5 μm to about 10 μm, or about 6μm to about 30 μm, or about 7 μm to about 30 μm, or about 8 μm to about30 μm, or about 10 μm to about 30 μm, or about 10 μm to about 25 μm, orabout 10 μm to about 20 μm, or about 15 μm to about 30 μm. In oneembodiment, one mode of pores has a mean pore diameter within the rangeof about 0.01 μm to about 5 μm (or any of the specific exemplary valuesgiven above within this range, or sub-ranges derived therefrom) whileanother mode of pores has a mean pore diameter above 5 μm up to about 30μm, or any of the sub-ranges given therein. In another embodiment, atleast two modes of pores have a mean pore diameter above 5 μm up toabout 30 μm.

A preferred bimodal distribution for the carrier is depicted in FIG. 1.The solid line in FIG. 1 shows the distribution of pore diameters in thetwo modes by plotting pore diameter against pore volume distribution.One mode of pores is shown to have a range of pore sizes within about0.1 to 2.0 μm while another mode of pores is shown to have a range ofpore sizes within about 0.5 or 1.0 to 5 μm. While the modes of pores areshown to significantly overlap in this instance, in other instances themodes of pores may overlap much less or not at all. The dashed line inFIG. 1 shows the pore diameter plotted against the logarithmicdifferential volume.

In a first embodiment, the first mode of pores comprises at most about50% of the total pore volume and the second mode of pores comprises atleast about 50% of the total pore volume. In a second embodiment, thefirst or second mode of pores comprises at most about 45% of the totalpore volume and the other mode of pores comprises at least about 55% ofthe total pore volume. In a third embodiment, the first or second modeof pores comprises at most about 40% of the total pore volume and theother mode of pores comprises at least about 60% of the total porevolume. In a fourth embodiment, the first or second mode of porescomprises at most about 35% of the total pore volume and the other modeof pores comprises at least about 65% of the total pore volume. In afifth embodiment, the first or second mode of pores comprises at mostabout 30% of the total pore volume and the other mode of pores comprisesat least about 70% of the total pore volume. Numerous other embodimentsreflective of different bimodal pore distributions are possible andwithin the scope of the present invention. Without wishing to be boundby any theory, it is believed that a catalyst with the described bimodalpore size distribution possesses a type of pore structure in whichreaction chambers are separated by diffusion channels. The pore volumeand pore size distribution described herein can be measured by anysuitable method, but are more preferably obtained by the conventionalmercury porosimeter method as described in, for example, Drake andRitter, Ind. Eng. Chem. Anal. Ed., 17, 787 (1945).

100341 Preferably, the mean pore diameter of the first mode of pores andthe mean pore diameter of the second mode of pores (i.e., the“differential in mean pore diameters”) are different by at least about0.1 μm. In different embodiments, the difference in mean pore sizes canbe at least, for example, 0.2 μm, or 0.3 μm, or 0.4 μm, or 0.5 μm, or0.6 μm, or 0.7 μm, or 0.8 μm, or 0.9 μm, or 1.0 μm, or 1.2 μm, or 1.4μm, or 1.5 μm, 1.6 μm, or 1.8 μm, or 2.0 μm, or 2.5 μm, or 3 μm, or 4μm, or 5 μm, or 6 μm, or 7 μm, or 8 μm, or 9 μm, or 10 μm, and up toabout 15, 20 or 30 μm.

In a preferred carrier, at least 40% (and typically at least 60%, andmore typically at least 80%) of the pore volume is due to pores withdiameters between 1 and 5 micrometers. The median pore diameter of thecarrier employed in the invention is typically between about 1 and 5micrometers, more typically between about 1 and 4.5 micrometers, andeven more typically between about 1 and 4 micrometers. The pore volumefrom pores with a diameter of 5 micrometers and above is typically lessthan about 0.20 ml/g, more typically less than about 0.10 ml/g, and evenmore typically less than about 0.05 ml/g. The pore volume from poreswith a diameter of 1 micrometer and less is typically less than about0.20 ml/g, more typically less than about 0.16 ml/g, and even moretypically, less than about 0.12 ml/g. In some embodiments, the waterpore volume can be from about 0.10 cc/g to about 0.80 cc/g, and moretypically from about 0.20 cc/g to about 0.60 cc/g. The pore volume andpore size distribution described herein can be measured by any suitablemethod, but are more preferably obtained by the conventional mercuryporosimeter method as described in, for example, Drake and Ritter, “Ind.Eng. Chem. Anal. Ed.,” 17, 787 (1945).

The carrier of the invention can be of any suitable shape or morphology.For example, the carrier can be in the form of particles, chunks,pellets, rings, spheres, three-holes, wagon wheels, cross-partitionedhollow cylinders, and the like, of a size preferably suitable foremployment in fixed bed reactors. Typically, carrier particles haveequivalent diameters in the range of from about 3 mm to about 12 mm, andmore typically in the range of from about 5 mm to about 10 mm, which areusually compatible with the internal diameter of the tubular reactors inwhich the catalyst is placed. As known in the art, the term “equivalentdiameter” is used to express the size of an irregularly-shaped object byexpressing the size of the object in terms of the diameter of a spherehaving the same volume as the irregularly-shaped object.

In one embodiment, the carrier of the invention contains essentiallyonly alumina and mullite components in the absence of other metals orchemical compounds, except that trace quantities of other metals orcompounds may be present. A trace amount is an amount low enough thatthe trace species does not observably affect functioning or ability ofthe catalyst.

In another embodiment, the carrier of the invention contains one or morepromoting species. As used herein, a “promoting amount” of a certaincomponent of a catalyst refers to an amount of that component that workseffectively to provide an improvement in one or more of the catalyticproperties of the catalyst when compared to a catalyst not containingsaid component. Examples of catalytic properties include, inter alia,operability (resistance to runaway), selectivity, activity, conversion,stability and yield. It is understood by one skilled in the art that oneor more of the individual catalytic properties may be enhanced by the“promoting amount” while other catalytic properties may or may not beenhanced or may even be diminished. It is further understood thatdifferent catalytic properties may be enhanced at different operatingconditions. For example, a catalyst having enhanced selectivity at oneset of operating conditions may be operated at a different set ofconditions wherein the improvement is exhibited in the activity ratherthan in the selectivity.

For example, the mullite-containing carrier described above may includea promoting amount of an alkali metal or a mixture of two or more alkalimetals. Suitable alkali metal promoters include, for example, lithium,sodium, potassium, rubidium, cesium or combinations thereof. Cesium isoften preferred, with combinations of cesium with other alkali metalsalso being preferred. The amount of alkali metal will typically rangefrom about 10 ppm to about 3000 ppm, more typically from about 15 ppm toabout 2000 ppm, more typically from about 20 ppm to about 1500 ppm, andeven more typically from about 50 ppm to about 1000 ppm by weight of thetotal catalyst, expressed in terms of the alkali metal.

The carrier of the invention may also include a promoting amount of aGroup IIA alkaline earth metal or a mixture of two or more Group IIAalkaline earth metals. Suitable alkaline earth metal promoters include,for example, beryllium, magnesium, calcium, strontium, and barium orcombinations thereof. The amounts of alkaline earth metal promoters areused in similar amounts as the alkali metal promoters described above.

The carrier of the invention may also include a promoting amount of amain group element or a mixture of two or more main group elements.Suitable main group elements include any of the elements in Groups IIIA(boron group) to VIIA (halogen group) of the Periodic Table of theElements. For example, the catalyst can include a promoting amount ofone or more sulfur compounds, one or more phosphorus compounds, one ormore boron compounds, one or more halogen-containing compounds, orcombinations thereof. The catalyst can also include a main groupelement, aside from the halogens, in its elemental form.

The carrier of the invention may also include a promoting amount of atransition metal or a mixture of two or more transition metals. Suitabletransition metals can include, for example, the elements from GroupsIIIB (scandium group), IVB (titanium group), VB (vanadium group), VIB(chromium group), VIIB (manganese group), VIIIB (iron, cobalt, nickelgroups), IB (copper group), and IIB (zinc group) of the Periodic Tableof the Elements, as well as combinations thereof. More typically, thetransition metal is an early transition metal, i.e., from Groups IIIB,IVB, VB or VIB, such as, for example, hafnium, yttrium, molybdenum,tungsten, rhenium, chromium, titanium, zirconium, vanadium, tantalum,niobium, or a combination, thereof

The carrier of the invention may also include a promoting amount of arare earth metal or a mixture of two or more rare earth metals. The rareearth metals include any of the elements having an atomic number of57-103. Some examples of these elements include lanthanum (La), cerium(Ce), and samarium (Sm).

The transition metal or rare earth metal promoters are typically presentin an amount of from about 0.1 micromoles per gram to about 10micromoles per gram, more typically from about 0.2 micromoles per gramto about 5 micromoles per gram, and even more typically from about 0.5micromoles per gram to about 4 micromoles per gram of total catalyst,expressed in terms of the metal.

All of these promoters, aside from the alkali metals, can be in anysuitable form, including, for example, as zerovalent metals or highervalent metal ions.

Of the promoters listed, rhenium (Re) is preferred as a particularlyefficacious promoter for ethylene epoxidation high selectivitycatalysts. The rhenium component in the catalyst can be in any suitableform, but is more typically one or more rhenium-containing compounds(e.g., a rhenium oxide) or complexes. The rhenium can be present in anamount of, for example, about 0.001 wt.% to about 1 wt.%. Moretypically, the rhenium is present in amounts of, for example, about0.005 wt. % to about 0.5 wt. %, and even more typically, from about 0.01wt. % to about 0.05 wt. % based on the weight of the total catalystincluding the support, expressed as rhenium metal.

In another aspect, the invention is directed to an ethylene epoxidationcatalyst produced from the carrier described above. In order to producethe catalyst, a carrier having the above characteristics is thenprovided with a catalytically effective amount of silver thereon and/ortherein. The catalysts are prepared by impregnating the carriers withsilver ions, compounds, complexes, and/or salts dissolved in a suitablesolvent sufficient to cause deposition of silver precursor compound ontoand/or into the carrier. The carrier can be impregnated and incorporatedwith rhenium and silver, along with any desired promoters, by any of theconventional methods known in the art, e.g., by excess solutionimpregnation, incipient wetness impregnation, spray coating, and thelike. Typically, the carrier material is placed in contact with thesilver-containing solution until a sufficient amount of the solution isabsorbed by the carrier. Preferably, the quantity of thesilver-containing solution used to impregnate the carrier is no morethan is necessary to fill the pore volume of the carrier. Infusion ofthe silver-containing solution into the carrier can be aided byapplication of a vacuum. A single impregnation or a series ofimpregnations, with or without intermediate drying, may be used,depending in part on the concentration of the silver component in thesolution. Impregnation procedures are described in, for example, U.S.Pat. Nos. 4,761,394, 4,766,105, 4,908,343, 5,057,481, 5,187,140,5,102,848, 5,011,807, 5,099,041 and 5,407,888, all of which areincorporated herein by reference. Known procedures for pre-deposition,co-deposition, and post-deposition of the various promoters can also beemployed.

Silver compounds useful for impregnation include, for example, silveroxalate, silver nitrate, silver oxide, silver carbonate, a silvercarboxylate, silver citrate, silver phthalate, silver lactate, silverpropionate, silver butyrate and higher fatty acid salts and combinationsthereof. The silver solution used to impregnate the carrier can containany suitable solvent. The solvent can be, for example, water-based,organic-based, or a combination thereof. The solvent can have anysuitable degree of polarity, including highly polar, moderately polar ornon-polar, or substantially or completely non-polar. The solventtypically has sufficient solvating power to solubilize the solutioncomponents. Some examples of water-based solvents include water andwater-alcohol mixtures. Some examples of organic-based solvents include,but are not limited to, alcohols (e.g., alkanols), glycols (e.g., alkylglycols), ketones, aldehydes, amines, tetrahydrofuran, nitrobenzene,nitrotoluene, glymes (e.g., glyme, diglyme and tetraglyme), and thelike, and their combinations. Organic-based solvents that have 1 toabout 8 carbon atoms per molecule are preferred.

A wide variety of complexing or solubilizing agents may be employed tosolubilize silver to the desired concentration in the impregnatingmedium. Useful complexing or solubilizing agents include amines,ammonia, lactic acid and combinations thereof. For example, the aminecan be an alkylene diamine having from 1 to 5 carbon atoms. In apreferred embodiment, the solution comprises an aqueous solution ofsilver oxalate and ethylene diamine. The complexing/solubilizing agentmay be present in the impregnating solution in an amount from about 0.1to about 5.0 moles of ethylene diamine per mole of silver, preferablyfrom about 0.2 to about 4.0 moles, and more preferably from about 0.3 toabout 3.0 moles of ethylene diamine for each mole of silver.

The concentration of silver salt in the solution is typically in therange from about 0.1% by weight to the maximum permitted by thesolubility of the particular silver salt in the solubilizing agentemployed. More typically, the concentration of silver salt is from about0.5% to 45% by weight of silver, and even more typically, from about 5to 35% by weight.

The ethylene oxide (EO) catalyst contains a catalytically effectiveamount of silver metal to catalyze the synthesis of ethylene oxide fromethylene and oxygen. The silver can be located on the surface and/orthroughout the pores of the refractory support. A catalyticallyeffective amount of silver can be, for example, up to about 45% byweight of silver, expressed as metal, based on the total weight of thecatalyst including the support. Silver contents, expressed as metal, offrom about 1% to about 40% based on the total weight of the catalyst aremore typical. In other embodiments, the silver content can be from, forexample, about 1 to 35%, 5 to 35%, 1 to 30%, 5 to 30%, 1 to 25%, 5 to25%, 1 to 20%, 5 to 20%, 8 to 40%, 8 to 35%, 8 to 30%, 10 to 40%, 10 to35%, 10 to 25%, 12 to 40%, 12 to 35%, 12 to 30%, or 12 to 25%.

Rhenium is also preferably incorporated into the silver-containingcatalyst in order to provide a high selectivity catalyst. The rhenium isincorporated in the promoting amounts described above either prior to(i.e., by prior incorporation into the carrier), coincidentally with, orsubsequent to the deposition of the silver.

Any one or more other promoting species can also be incorporated intothe carrier either prior to, coincidentally with, or subsequent to thedeposition of the silver. In one preferred embodiment, additionalpromoters include one or more species selected from Cs, Li, W, and S. Inanother preferred embodiment, additional promoters include one or morespecies selected from Cs, Li, and S.

After impregnation with silver and any promoters, the impregnatedcarrier is removed from the solution and calcined for a time sufficientto reduce the silver component to metallic silver and to remove volatiledecomposition products from the silver-containing support. Thecalcination is typically accomplished by heating the impregnatedcarrier, preferably at a gradual rate, to a temperature in a range ofabout 200° C. to about 600° C., more typically from about 200° C. toabout 500° C., more typically from about 250° C. to about 500° C., andmore typically from about 200° C. or 300° C. to about 450° C., at areaction pressure in a range from about 0.5 to about 35 bar. In general,the higher the temperature, the shorter the required calcination period.A wide range of heating periods have been described in the art for thethermal treatment of impregnated supports. See, for example, U.S. PatentNo. 3,563,914, which indicates heating for less than 300 seconds, andU.S. Patent No. 3,702,259, which discloses heating from 2 to 8 hours ata temperature of from 100° C. to 375° C. to reduce the silver salt inthe catalyst. A continuous or step-wise heating program may be used forthis purpose.

During calcination, the impregnated support is typically exposed to agas atmosphere comprising an inert gas, such as nitrogen. The inert gasmay also include a reducing agent.

In another aspect, the invention is directed to a method for the vaporphase production of ethylene oxide by conversion of ethylene to ethyleneoxide in the presence of oxygen by use of the catalyst described above.Generally, the ethylene oxide production process is conducted bycontinuously contacting an oxygen-containing gas with ethylene in thepresence of the catalyst at a temperature in the range from about 180°C. to about 330° C., more typically from about 200° C. to about 325° C.,and more typically from about 225° C. to about 270° C., at a pressurewhich may vary from about atmospheric pressure to about 30 atmospheresdepending on the mass velocity and productivity desired. Pressures inthe range of from about atmospheric to about 500 psi are generallyemployed. Higher pressures may, however, be employed within the scope ofthe invention. Residence times in large-scale reactors are generally onthe order of about 0.1 to about 5 seconds. A typical process for theoxidation of ethylene to ethylene oxide comprises the vapor phaseoxidation of ethylene with molecular oxygen in the presence of theinventive catalyst in a fixed bed, tubular reactor. Conventionalcommercial fixed bed ethylene oxide reactors are typically in the formof a plurality of parallel elongated tubes (in a suitable shell). In oneembodiment, the tubes are approximately 0.7 to 2.7 inches O.D. and 0.5to 2.5 inches I.D. and 15-45 feet long filled with catalyst.

The inventive catalysts have been shown to be particularly selectivecatalysts in the oxidation of ethylene with molecular oxygen to ethyleneoxide. The conditions for carrying out such an oxidation reaction in thepresence of the catalyst of the present invention broadly comprise thosedescribed in the prior art. This applies, for example, to suitabletemperatures, pressures, residence times, diluent materials (e.g.,nitrogen, carbon dioxide, steam, argon, methane or other saturatedhydrocarbons), the presence or absence of moderating agents to controlthe catalytic action (e.g., 1, 2-dichloroethane, vinyl chloride or ethylchloride), the desirability of employing recycle operations or applyingsuccessive conversion in different reactors to increase the yields ofethylene oxide, and any other special conditions which may be selectedin processes for preparing ethylene oxide. Molecular oxygen employed asa reactant may be obtained from conventional sources. The suitableoxygen charge may be relatively pure oxygen, or a concentrated oxygenstream comprising oxygen in a major amount with lesser amounts of one ormore diluents such as nitrogen or argon, or air.

In the production of ethylene oxide, reactant feed mixtures typicallycontain from about 0.5 to about 45% ethylene and from about 3 to about15% oxygen, with the balance comprising comparatively inert materialsincluding such substances as nitrogen, carbon dioxide, methane, ethane,argon and the like. Only a portion of the ethylene is typically reactedper pass over the catalyst. After separation of the desired ethyleneoxide product and removal of an appropriate purge stream and carbondioxide to prevent uncontrolled build up of inert products and/orby-products, unreacted materials are typically returned to the oxidationreactor. For purposes of illustration only, the following are conditionsthat are often used in current commercial ethylene oxide reactor units:a gas hourly space velocity (GHSV) of 1500-10,000 h⁻¹, a reactor inletpressure of 150-400 prig, a coolant temperature of 180-315° C., anoxygen conversion level of 10-60%, and an EO production (work rate) of100-300 kg EO per cubic meters of catalyst per hour. Typically, the feedcomposition at the reactor inlet comprises 1-40% ethylene, 3-12% oxygen,0.3-40% CO₂, 0-3% ethane, 0.3-20 ppmv total concentration of organicchloride moderator, and the balance of the feed comprised of argon,methane, nitrogen, or mixtures thereof.

In other embodiments, the process of ethylene oxide production includesthe addition of oxidizing gases to the feed to increase the efficiencyof the process. For example, U.S. Pat. No. 5,112,795 discloses theaddition of 5 ppm of nitric oxide to a gas feed having the followinggeneral composition: 8 volume % oxygen, 30 volume % ethylene, about 5ppmw ethyl chloride, and the balance nitrogen.

The resulting ethylene oxide is separated and recovered from thereaction products using methods known in the art. The ethylene oxideprocess may include a gas recycle process wherein a portion orsubstantially all of the reactor effluent is readmitted to the reactorinlet after substantially or partially removing the ethylene oxideproduct and any byproducts. In the recycle mode, carbon dioxideconcentrations in the gas inlet to the reactor may be, for example, fromabout 0.3 to about 6 volume percent.

Examples have been set forth below for the purpose of furtherillustrating the invention. The scope of this invention is not to be inany way limited by the examples set forth herein.

COMPARATIVE EXAMPLE 1

HAC Catalysts Prepared on Mullite-Free and Mullite-Containing Supports

An HAC catalyst was prepared on a mullite-free alpha-alumina support ata silver concentration, i.e., [Ag], of 11.6% and cesium concentration,i.e., [Cs], of 472 ppm.

A separate HAC catalyst was prepared on a mullite-containing (ca. 9% byweight mullite) alpha-alumina carrier with [Ag]=11.7% and [Cs]=440 ppm.

The two HAC catalysts were then subjected to an accelerated aging testat weight work rate (WWR)=737 g EO per 1 kg catalyst per 1 hour.

The performance results for the catalyst are shown below in Table 1. Asshown in the table, the two HAC catalysts exhibited the same change inselectivity over 1000 hours (i.e., ΔS_(1ooo h)=0) regardless of whetherthe carrier included or excluded mullite. Accordingly, it is evidentthat a conventional HAC catalyst does not require stability enhancement.

TABLE 1 Performance of HACs in an accelerated evaluation test CarrierS_(SOR) ¹ ΔS_(1000 h) No mullite 82.2 0 9% mullite 82.3 0 ¹Start of Run(SOR) selectivity is measured after activation of catalyst to targetwork rate

EXAMPLE 2

HSC Catalysts Prepared on Mullite-Free and Mullite-Containing Supports

The HSC catalysts described below are based on alpha-alumina carrierscontaining the following promoters: Cs (as CsOH), Li (as LiNO₃), Re (asHReO₄), W (as ammonium metatungstate), and S (as ammonium sulfate).Promoter concentrations were optimized to provide maximum stability athigh selectivity and were within the ranges found within examples 3-10through 7-20 of U.S. Pat. No. 4,766,105.

An HSC catalyst was prepared on a mullite-free alpha-alumina carrierhaving the above promoter composition and with [Ag]=11.7%. This catalystis herein referred to as catalyst HSC-1.

A separate HSC catalyst was prepared on a mullite-containing (ca. 9% byweight mullite) alpha-alumina carrier having the above promotercomposition and with [Ag]=14.5%. This catalyst is herein referred to ascatalyst HSC-2.

A separate HSC catalyst was prepared on a mullite-containing (ca. 9% byweight mullite) alpha-alumina carrier having the above promotercomposition and with [Ag]=16.5%. This catalyst is herein referred to ascatalyst HSC-3.

The three HSC catalysts were then subjected to an HSC accelerated agingtest at weight work rate (WWR)=540 (g EO per 1 kg catalyst per 1 hour).The performance results for the HSC catalysts are shown below in Table2. As shown in the table, the HSC catalyst not containing mullite(HSC-1) exhibits a change in selectivity over 1000 hours (ΔS_(1000 h))of 4.6. In contrast, it has been surprisingly found that the two HSCcatalysts containing mullite (i.e., HSC-2 and HSC-3) exhibit,respectively, a significantly reduced ΔS_(1000 h) of <0.3 and 0.8,respectively. Therefore, it is evident from the data shown above thatthe mullite-containing HSC catalyst of the invention is significantlyimproved in stability, and hence, longevity, by an improved retention ofselectivity as compared to HSC catalysts of the prior art over the sametime period of operation.

TABLE 2 Performance of HSCs in an accelerated evaluation test CarrierS_(MAX) ² S_(AV1500 h) ³ ΔS_(1000 h) No mullite (HSC-1) 89.5 86.9 4.6 9%mullite (HSC-2) 88.5 88.2 <0.3 9% mullite (HSC-3) 89.8 89.2 0.8 ²S_(MAX)is maximum measured selectivity after reaching the target work rate.³S_(AV1500 h) is calculated average selectivity in the 1500 hour test attarget work rate.

The inventors have made the surprising and unexpected discovery that,whereas HAC catalysts do not exhibit a beneficial effect fromincorporation of mullite, HSC catalysts show a pronounced improvement inretention of selectivity, and hence, usable lifetime of the catalystwhen mullite is incorporated into their carriers at the sameconcentration.

While there have been shown and described what are presently believed tobe the preferred embodiments of the present invention, those skilled inthe art will realize that other and further embodiments can be madewithout departing from the spirit and scope of the invention describedin this application, and this application includes all suchmodifications that are within the intended scope of the claims set forthherein.

What is claimed is:
 1. An ethylene epoxidation catalyst comprising: a) acarrier comprising alumina in combination with a stability-enhancingamount of mullite; b) a catalytic amount of silver deposited on and/orin said carrier; and c) a promoting amount of rhenium deposited onand/or in said carrier.
 2. The catalyst according to claim 1, whereinthe alumina is a-alumina.
 3. The catalyst according to claim 1, whereinthe stability-enhancing amount of mullite is about 0.5-20% mullite. 4.The catalyst according to claim 1, wherein the stability-enhancingamount of mullite is about 1-15% mullite.
 5. The catalyst according toclaim 1, wherein the stability-enhancing amount of mullite is about1-12% mullite.
 6. The catalyst according to claim 1, wherein thestability-enhancing amount of mullite is about 3-15% mullite.
 7. Thecatalyst according to claim 1, wherein the stability-enhancing amount ofmullite is about 3-12% mullite.
 8. The catalyst according to claim 1,further comprising a promoting amount of an alkali or alkaline earthmetal.
 9. The catalyst according to claim 1, further comprising apromoting amount of cesium.
 10. The catalyst according to claim 1,further comprising a promoting amount of tungsten.
 11. The catalystaccording to claim 1, further comprising a promoting amount of sulfur.12. The catalyst according to claim 1, further comprising a promotingamount of cesium, lithium, tungsten, and sulfur.
 13. The catalystaccording to claim 1, further comprising a promoting amount of cesium,lithium, and sulfur.
 14. The catalyst according to claim 1, wherein thecarrier possesses pores having diameters of at least about 0.01 μm andup to about 5 μm.
 15. The catalyst according to claim 1, wherein thecarrier possesses a bimodal distribution of pore sizes comprising afirst and a second distribution of pore sizes, wherein each distributionof pore sizes possesses a different mean pore size.
 16. The catalystaccording to claim 15, wherein at least one distribution of pore sizespossesses a mean pore size within the range 0.01-5
 17. The catalystaccording to claim 15, wherein the first and second distribution of poresizes each possesses a mean pore size within the range 0.01-5 μm.
 18. Amethod for the vapor phase conversion of ethylene to ethylene oxide inthe presence of oxygen, the method comprising reacting a reactionmixture comprising ethylene and oxygen in the presence of a catalystcomprising: a) a carrier comprising alumina in combination with astability-enhancing amount of mullite; b) a catalytic amount of silverdeposited on and/or in said carrier; and c) a promoting amount ofrhenium deposited on and/or in said carrier.
 19. The method according toclaim 18, wherein the alumina is a-alumina.
 20. The method according toclaim 18, wherein the stability-enhancing amount of mullite is about0.5-20% mullite.
 21. The method according to claim 18, wherein thestability-enhancing amount of mullite is about 1-15% mullite.
 22. Themethod according to claim 18, wherein the stability-enhancing amount ofmullite is about 1-12% mullite.
 23. The method according to claim 18,wherein the stability-enhancing amount of mullite is about 3-15%mullite.
 24. The method according to claim 18, wherein thestability-enhancing amount of mullite is about 3-12% mullite.
 25. Themethod according to claim 18, further comprising a promoting amount ofan alkali or alkaline earth metal.
 26. The method according to claim 18,further comprising a promoting amount of cesium.
 27. The methodaccording to claim 18, further comprising a promoting amount oftungsten.
 28. The method according to claim 18, further comprising apromoting amount of sulfur.
 29. The method according to claim 18,wherein the carrier possesses pores having diameters of at least about0.01 μm and up to about 5 μm.
 30. The method according to claim 18,wherein the carrier possesses a bimodal distribution of pore sizescomprising a first and a second distribution of pore sizes, wherein eachdistribution of pore sizes possesses a different mean pore size.
 31. Themethod according to claim 30, wherein at least one distribution of poresizes possesses a mean pore size within the range 0.01-5 μm.
 32. Themethod according to claim 30, wherein the first and second distributionof pore sizes each possesses a mean pore size within the range 0.01-5μm.